Inleads containing a thin foil portion of a refractory metal such as tungsten or molybdenum have been commonly used for sealing into quartz envelopes to provide current conductors to the electrodes. These metals can withstand the very high temperatures necessary for sealing into quartz. Provided the foil or ribbon portions are sufficiently thin, they will merely go into tension when the bulb cools but will not rupture nor crack the seals. The inlead may be composite comprising a length of foil with a wire welded to each end, or it may be made from a single piece of metal, for instance by rolling a wire between pressure rolls as taught in U.S. Pat. No. 2,667,595--Noel et al., 1954.
The electrode inlead assemblies used in high pressure discharge lamps generally comprise an inlead of the foregoing kind having an electrode structure formed on one end, as by winding a tungsten wire around the shank portion. An arc tube comprises one such assembly sealed into each end of a quartz tube. The common method of sealing has been to stand the electrode inlead assembly up on a spindle, place one end of the quartz tube around it, heat the quartz to softening temperature, and then pinch or press the end of the tube shut between a pair of opposed fast acting jaws. Reference may be made to U.S. Pat. No. 2,965,698--Gottschalk, 1960, for a fuller description of pinch sealing.
The foliated portion of the inlead must be very thin in order to avoid shaling off and remain hermetically bonded to the quartz. Thicknesses greater than 0.0015" may give trouble with leaks and a thickness of 0.0009" at the thickest portion of the foil is typical. The result has been that the inlead is lacking in stiffness and bends so readily that horizontal sealing has been impractical. Vertical pinch-sealing has been the rule. However when an electrode inlead assembly is mounted on a spindle preparatory to sealing, as shown for instance at 6 in FIG. 2 of the Gottschalk patent, it can barely stand vertical and the electrode portion frequently leans and sags over to one side or the other. In conventional vertical pinch sealing, lack of inlead stiffness is not too important; if the electrode should lean over, the forceful movement of the viscous quartz by the pressing jaws snaps the electrodes substantially back into place at pinching. Furthermore, in prior art high pressure metal vapor lamps which generally were rated in excess of 100 watts, the arc gap or distance between the electrode tips would be several centimeters. In such lamps a misplacement of the electrodes in the end by a millimeter or so would have no appreciable effect on the electrical characteristics and performance of the lamp.
In electric lamp manufacture, the arc voltage drop is an important parameter which must be kept constant but it varies proportionally to the length of the interelectrode gap. Accordingly, as the size of lamp and length of gap are reduced, the need for accuracy in gap determination increases in importance. Also the heating of the ends of the arc chamber is strongly influenced by the extent to which the electrodes are inserted and project into the arc chamber. Such heating determines the extent of vaporization of the fill, particularly of the metal halides which tend to condense in the cooler ends. Thus both the length and the location of the inter-electrode gap are important and the need for precision in its determination increases as the size of the lamp is reduced.
In copending application Ser. No. 912,268, filed June 5, 1978 by Cap and Lake, entitled "High Pressure Metal Vapor Discharge Lamps of Improved Efficacy" and which is assigned to the same assignee as the present invention, new lamp designs are disclosed utilizing shaped envelopes with small end seals for reducing end losses. In these new lamps and particularly in the smaller sizes having arc chamber volumes less than 1 cubic centimeter, precision in both the length of the interelectrode gap and its location within the bulb is essential.